Why is the sky blue?
This is a very big question, but I am a geek and a know it.
If you are referring to the thread pattern, then it is Metric. BMW started using Metric sizes after the E30 BMW's.
The bolt head are all 6 sided.
Most bolts on the panel work will be sized between 10 - 13mm, plastic panels (bumpers, engine under tray) have a mix of plastic clips and 13mm bolts and female Trox.
Inside the car, it changes slightly, you have a massive range of different plastic clips, 12mm bolts and TROX screws from size T10 - T25.
I hope this is what you were looking for.